Quality assurance is an important aspect in the manufacture of industrial products and also, for example, in the production and preparation of food in the food industry. Consumers expect only good quality products to be processed and offered for sale. Products of a lesser quality must therefore be removed from the distribution, production or preparation process at an earlier stage.
Traditionally, quality inspections are predominantly carried by human experts. In many cases the inspection is a visual inspection. The expert visually examines the products and decides which of the inspected products comply or do not comply with predetermined quality criteria. For example, which products are fit for sale in shops or eligible for further processing, or which products must be declared unfit. The outcome of such an assessment not only exhibits a wide variation between the various human experts, but it also varies from day to day, for example, and depends on the rate at which the products are being brought in for inspection. An additional advantage in this regard is that the work is labour-intensive and tiring, and that there is a great risk of errors being made. This is the case in particular when inhomogeneous products are being inspected.
In practice, automated sorting devices are used, which, making use of optical systems, assess whether products meet predetermined quality requirements.
The products to be sorted or separated are placed in bulk on a conveyor belt and carried to an inspection zone, where they are inspected for abnormalities and defects by the detection system. The results of the detection process are fed to a digital processing unit, which decides which products meet predetermined criteria, such as quality requirements. The products to be separated, i.e. the products that comply with or, on the contrary, do not comply with the relevant criteria, are then separated from the bulk stream of products on the conveyor belt by suitably controlling the separating device.
In order to be able to correctly determine the various properties of a product, an optical camera system may be made up of several cameras. Line scanning cameras are used, for example, which are capable of high-speed registration of (line) images of the products in the bulk stream over a limited field of view for the purpose of determining visual aspects of the products from these images. Besides cameras which register images in the spectrum visible to the human eye, also Near Infrared (NIR) cameras are used for determining properties of the products to be sorted that are not visible to the human eye, for example internal properties.
In practice, separating means based on the so-called flow principle, among other means, are used for separating products from the bulk stream. These means comprise several valves or nozzles arranged across the width of the conveyor belt at the end of the conveyor belt. When a product to be separated is present near a valve, the valve in question or a number of valves are actuated and an air blast is directed towards the product to be separated via this valve or these valves. The product to be separated is then “blasted” from the bulk stream of products by means of this air blast.
Another known manner of separating products from a bulk stream of products is to use mechanical fingers or pins or the like. These fingers or pins can for example be set to deflect mechanically products to be separated from the bulk stream of products. The products to be separated are thus sorted from the bulk stream of products. Still further separating means may comprise grippers for picking up products from the bulk stream and removing the products.
When used in the food industry, for example for evaluating and sorting potato parts in the form of french fries, it must be possible, using automated sorting devices, to process an amount of product in the order of 10,000-18,000 kg/h per sorting device. A usual width of the conveyor belt is 200-250 cm. The french fries are conveyed past the camera system toward the separating means at a speed of about 1.5-3 m/sec in the sorting device. This implies that such a sorting device must have sufficient computing power for determining within a relatively short period of about 5-20 msec per product from the bulk stream which products are to be separated and for controlling the separating means to sort the products.
A drawback of the sorting devices that are known in practice is inter alia the fact that the effectiveness of the sorting process is insufficient when products are to be separated from a random or non-ordered bulk stream of inhomogeneous products. The term “inhomogeneous products” is understood to comprise products of the same type or kind, which may strongly differ from each other as regards their properties. Think in this regard of visually perceptible differences such as colour, shape, dimensions, etc. and in the case of food products, for example, also of internal product differences such as ripeness, hardness, water content, fat content, etc. The term “random” is understood to comprise those products that are present on top of each other or next to each other in the bulk stream, in a non-ordered manner.
Defective products or unsatisfactory products, i.e. products which do not meet predetermined quality criteria, in a random bulk stream of inhomogeneous products are not adequately separated by the current automated sorting products. As a result, an unacceptably large proportion of defective or below-standard products will eventually remain among those products that have been evaluated as having the required quality, for example for sale, for further processing or for being prepared into a final product. On the other hand it also happens, of course, that products which are not defective or which do have the required quality are undeservedly separated from the bulk stream. This is undesirable, since satisfactory products may thus be discarded as waste or be sold at a price which is too low.
The efficiency with which the current sorting devices separate products from a random bulk stream of inhomogeneous products is found to be insufficient in practice.